This invention generally relates to particulate resin compositions suitable for production of high-resolution toners for developing latent electrostatic images in electrophotography, electrostatic recording and electrostatic printing. More specifically, this invention relates in preferred embodiments to a dispersion comminution method of forming suitably sized resin particles which may be converted to a particulate toner composition for high-resolution electrophotography, electrostatic recording and electrostatic printing by incorporating a coloring agent and other suitable components therein.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process (U.S. Pat. No. 2,297,691) involves placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic toner material. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image subsequently may be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or over coating treatment.
Also well known are techniques to develop such electrostatic images. Developer is a vehicle in which are dispersed charged colored toner particles. The photoreceptor bearing the electrostatic latent image is contacted with the developer. The contact causes the charged toner particles in the developer to migrate to the charged areas of the photoreceptor to develop the latent image. Then, the photoreceptor is developed with the charged colored particles adhering to the latent image in image configuration. The developed image is then typically transferred to a suitable substrate, such as paper or transparency material, and optionally may be fixed to the substrate by heat, pressure or other suitable means.
Toners and developer compositions including colored particles are well known. In this regard, see U.S. Pat. Nos.: 5,352,521; 4,778,742; 5,470,687; 5,500,321; 5,102,761; 4,645,727; 5,437,953; 5,296,325 and 5,200,290 the disclosures of which are hereby incorporated by reference. The traditional compositions normally contain toner particles consisting of resin and colorants, wax or a polyolefin, charge control agents, flow agents and other additives. A typical toner formulation generally contains about 90-95 weight percent resin, about 2-10 weight percent colorant, from about 0 to about 6 weight percent wax, from about 0 to about 3 weight percent charge control agent, about 0.25-1 weight percent flow agent and from about 0 to about 1 weight percent other additives. Major resins are styrene-acrylic copolymers, styrene-butadiene copolymers and polyesters. The colorants usually are selected from cyan dyes or pigments, magenta dyes or pigments, yellow dyes or pigments, and mixtures thereof.
Conventional color toner particles are produced by a milling process described, for example, in the aforementioned U.S. Pat. No. 5,102,761. In that process, a polyacrylate resin is compounded with pigments, charge control agents (xe2x80x9cCCAxe2x80x9d) and occasionally wax in a melt mixer. The resulting polymer mixture is mechanically crushed and then milled into small particles. The conventional toner particles typically have an irregular shape and a broad distribution in particle size. For optimum resolution of images and color, smaller particles perform better. Thus, for example, it is difficult to obtain resolutions better than about 600 dots/inch when the average particle size is more than about 7 xcexcm. For resolutions in the order of about 1200 dots/inch, particle sizes smaller than 5 xcexcm are typically needed. It is difficult to make particles smaller than about 7-10 xcexcm by conventional processes because of the high energy cost of producing small particles as well as uniform narrow particle size distribution.
Many previous attempts to produce small toner particles with the size smaller than 7 xcexcm have been made. For example, the aforementioned U.S. Pat. Nos. 5,352,521, 5,470,687 and 5,500,321 disclose toner particles produced by dispersion polymerization. In such a method, monomers (typically styrenic and acrylate monomers) and additives such as pigments, CCA and wax are mixed together to form a dispersion. This is then further dispersed into an aqueous or a non-aqueous medium and the monomers are polymerized to form toner particles. This method has the advantage over other methods that spherical toner particles with a small diameter can be prepared by a single process. However, the polymerization involves a substantial volume contraction and it results in entrapment of the dispersion medium inside the toner particles. Furthermore, the polymerization is difficult to be brought to completion and a substantial portion of the monomers remains in the toner particles. The residual monomers and the entrapped dispersion solvent are difficult to separate from the particles. Also, the polarity of the polymerizing materials changes drastically in the course of the polymerization and the additives tend to exude from the particle bulk and tend to concentrate on the surface thereof. Further, agents employed, such as dispersion-stabilizing agent and surface active agent, which cause the charging characteristics and preservability of the toner particles to deteriorate, remain on the surface of the toner particles, and those agents are extremely difficult to remove from the toner particles. Some methods have included the suggestion of dispersing polymer/solvent droplets in a water medium and shearing the mixture. However, water tends to get into the interstices between particles and agglomerate them. Once agglomeration occurs, it is very difficult to drive off the water without damaging or otherwise altering the physical properties of the particles, especially with respect to polymers having relatively low softening points, that is below about 100xc2x0 C.
A co-pending application, U.S. patent application Ser. No. 09/571,772 discloses a method of producing toner particles by comminuting resin particles comprising a colorant and a charge control agent in a solvent which does not dissolve the resin. However, applicability of the method is somewhat limited to toner resins with a relatively low molecular weight and the method generally requires a moderately high temperature and a vigorous shearing for effective comminution of toner particles. Furthermore, the toner particles produced by the method typically have a smooth surface texture and tend to lack fast triboclectric charging characteristics which is important in mono-component electrophotography development systems.
Co-pending U.S. patent application Ser. No. 09/860,959 entitled MICRO-SERRATED COLOR TONER PARTICLES AND METHOD OF MAKING SAME, discloses an improved method of producing toner particles by an improved dispersion comminution method of producing high-resolution color toner which has a superior combination of properties for electrophotographic imaging systems. The process includes forming spherical toner particles with a small diameter distribution by way of dispersing a polymer resin compounded with a colorant, a vaporizable plasticizer component and other additives in a dispersion medium including a surfactant under shearing conditions. The method may be carried out at a substantially low temperature compared to the method disclosed aforementioned U.S. patent application Ser. No. 09/571,772 and allows a resin with a high molecular weight to be used for preparing a toner composition. Furthermore, the toner particles produced by the method tend to have a narrow size distribution. Also, the toner particles can be made to have a rough surface texture and thus to have a fast charging characteristics.
The above-cited references generally describe methods of producing toner compositions by first blending all constituents comprising a toner composition and subsequently forming a particulate toner composition either by a milling, a polymerization or a comminution process. Another approach proposed for producing a toner composition is to prepare a particulate resin composition and hen subject the composition to a process of incorporating a colorant and other toner additives. Such an approach to produce a toiler is disclosed in U.S. Pat. No. 6,001,524, which is incorporated by reference. The ""524 patent discloses polyester toner particles which are produced by incorporating a dye and a charge control agent into polyester resin particles. The resin particles are produced by a non-aqueous dispersion polymerization of suitable monomers. This method has the advantage of that spherical toner particles with a small diameter can be prepared. However, the polymerization involves a substantial volume contraction and it results in entrapment of the dispersion medium inside the toner particles. The entrapped dispersion solvent is difficult to separate from the particles and tend to produce a foggy image when printed with the toner. Furthermore, the toner particles tend to have a smooth surface texture. The smooth surface texture and the entrapped dispersion solvent tend to make the charge generation in these particles too slow for use in mono-component electrophotography systems.
Another desirable property in a particulate toner composition is a narrow particle size distribution. It is generally believed that a narrow size distribution leads to a more uniform charge distribution in the toner composition which. In turn, leads to a better line resolution in a printed image as well as reduction in spotty background. The conventional milling method of producing toner particles is generally inefficient in producing particles with a narrow size distribution and therefore has to employ a classification step to remove particles that are too small or too large from the toner composition.
Narrowness of the size distribution may be expressed by the 80% span (the span). The span is defined as the ratio of the size range in which middle 80% by volume of the particles occupy to the median size. A more detailed description of the definition of the span is in a later section on the characterization methods used in the present invention. A smaller value of the span therefore means a narrower size distribution. The span value of a typical toner composition which is commercially available by way of a conventional classification step is about 1.2. A method of toner particle formation yielding particles with the span value less than 1.2 without a classification process is highly desirable.
There is continuing interest in the development of new and improved methods of producing toners for application in high-resolution electrophotography.
Accordingly, an object of the present invention is to provide an improved dispersion comminution method of producing resin particles suitable for production of high-resolution color toner by forming spherical resin particles with a small diameter and a narrow size distribution by way of dispersing a polymer resin compounded with a vaporizable plasticizer component and an optional charge control agent in a dispersion medium including a surfactant under a shearing condition.
Another object of the present invention is to provide an improved dispersion comminution method of producing finely divided resin particles herein the comminution process may be carried out at a low temperature.
Yet another object of the present invention is to provide an improved dispersion comminution method of producing resin particles wherein a polymer resin with a relatively high molecular weight may be expeditiously comminuted.
Still another object of the present invention is to provide a method of producing resin particles comprising a polymer resin and optionally a charge control agent, which are substantially spherical in shape with a diameter in the range of about 1 to 10 xcexcm as well as a narrow particle size distribution.
A further object of the present invention is to provide resin particles suitable for production of high-resolution color toner which are substantially spherical in shape and have a serrated surface texture exhibiting fast charging characteristics.
Still other objects and advantages of the present invention shall become apparent from the accompanying description, examples and Figures.
There is provided in accordance with the present invention a particulate toner composition including resin particles containing a resin component and optionally a charge control agent characterized in that the resin particles have a micro-serrated surface exhibiting a surface roughness index of greater than about 1.2. Roughness indices of greater than about 1.5 or 2 are believed readily achieved if so desired.
Typically, the resin component has a weight average molecular weight, Mw, of from about 3,000 to about 100,000.
In another aspect of the present invention there is provided a process for preparing a particulate resin composition for production of high-resolution color toner for developing latent electrostatic images including the steps: a) preparing a first resin composition containing a resin component, an optional charge control agent and a vaporizable plasticizer component which reduces the melt viscosity of the resin composition and thereby facilitates the overall comminution process of this invention; b) dispersing the resin composition in an organic medium comprising a surfactant, wherein the resin component is substantially insoluble in the organic medium; c) comminuting the resin composition to form particulate resin particles by application of shear at an elevated temperature; d) removing the vaporizable plasticizer component by evaporation by maintaining the dispersion of particulate resin composition in the medium at an elevated temperature; e) recovering the resin particles using a filtration process, followed by washing with an organic solvent with a low boiling temperature and subsequently drying the particles. Without intending to be bound by any theory, it is believed that the micro-serrated structure of the particles is imparted to them during removal of the vaporizable plasticizer.
In a preferred aspect, the particulate resin composition comprises a polymer resin, and an optional charge control agent. The resin particles are substantially spherical in shape and have a volume average diameter in the range of from about 1 to about 10 microns. Furthermore, the resin particles have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8. A particularly desirable and surprising aspect of the present invention is that the resin particles may be made to have an irregular micro-serrated surface texture that increases the surface area. Toner particles made from the resin particles will have substantially improved triboelectric charging characteristics such as charging speed. A fast triboelectric charging characteristic of a toner composition is particularly important when the toner composition is used in a mono-component development systems which are widely employed in desktop laser printers.
Any suitable polymer resin may be employed as the resin component of the present invention. Particularly preferred resins include polyester resins and styrene copolymer resins. The polymer resin is typically an amorphous resin with a glass transition temperature in the range of from about 40xc2x0 C. to about 90xc2x0 C. The use of a vaporizable plasticizer component in the present method of producing resin particles significantly increases the molecular weight range of polymer resin usable for toner application. A desirable molecular weight range of a polymer resin processable with the method of the present invention is a weight average molecular weight in the range of from about 3000 g/mol to about 100,000 g/mol. The resin may preferably contain functional moieties which improves the compatibility with functionalized dyes as a part of its polymer chain chemical structure.
The resin may have functional sites in its polymer chain structure suitable for interacting with a functionalized dye selected from the group consisting of: hydroxyl moieties; alkoxyl moieties; sulfonic or derivatized sulfonic moieties; sulfonic or derivatized sulfonic moieties; carboxyl or derivatized carboxyl moieties; phosphonic or derivatized phosphonic moieties; phosphinic or derivatized phosphinic moieties; thiol moieties, amine moieties; alkyl amine moieties; quatemized amine moieties; and mixtures thereof.
The first resin composition is typically prepared by melt compounding the resin component with an optional charge control agent and the vaporizable plasticizer component. The charge control agent may be dispersed in the resin and may be a positive charge control agent or a negative charge control agent.
Presence of the vaporizable plasticizer component significantly reduces the melt viscosity and the flow temperature of the first resin composition and therefore allows the whole particle preparation process to be carried out at a substantially lower temperature than the process without a vaporizable plasticizer component. The vaporizable plasticizer component is selected from organic solvents which are absorbable in the polymer resin component and have a boiling temperature less than 200xc2x0 C. It is preferable that the vaporizable plasticizer component is insoluble in the organic solvent component employed in the dispersion preparation and comminution steps of the present invention. Preferred examples of the vaporizable plasticizer components are acetone, tetrahydrofuran, 1,2-dichloroethane, 1-methyl-2-pyrrolididone, dimethylformamide, cyclohexanone, dimethylsulfoxide, chirobenzene. The first resin composition may be prepared by melt compounding at a temperature which is determined by the choice and the amount of vaporizable plasticizer component in the first resin composition. It is preferable to carry out the preparation of the first resin composition at as low a temperature as feasible, however.
The first resin composition is dispersed in the immiscible organic medium by subjecting the mixture of the molten resin composition and the organic medium to a mild shear at an elevated temperature. Any suitable mixing equipment may be employed for this step. An example of such equipment is a vessel equipped with an impeller-type agitator and a means of heating the content of the vessel. Effective formation of dispersion as well as successful comminution requires that the solubility parameter of the organic medium be generally different from the solubility parameter of the resin component by at least about 1. In preferred embodiments the solubility parameter of the organic medium is larger or smaller than the solubility parameter of the resin component by at least about 2. Any suitable organic medium which does not dissolve the resin component may be employed. Particularly preferred solvents include paraffin solvents, water and poly (ethylene glycol).
The organic medium includes a surfactant which may be a non-ionic, a cationic or an anionic surfactant. Preferred examples of such surfactants include copolymers of vinylpyrrolidonone, alkylated maleic acid copolymers, polymers containing ethylene oxide moieties, polymers containing propylene oxide moieties and sodium dodecylsulfate. The surfactant is generally present in the organic medium in an amount from about 0.2 to about 15 weight percent based on the amount of solvent present whereas from about 1 to about 10 weight percent based on the amount of solvent present is typical.
The first resin composition is generally from about 10 to about 70 weight percent of the combined weight of the resin composition in the organic medium during the step of dispersing the first resin composition. From about 20 to about 50 weight percent of the combined weight of the first resin composition in the organic medium is more typical. The first resin composition may be introduced to the organic medium maintained at an elevated temperature under a shearing condition. Equally preferably, the organic medium may be introduced to molten first resin composition maintained at an elevated temperature under a shearing condition. During the step of dispersing the first resin composition, the organic medium is maintained at an elevated temperature. The temperature may be selected to be any value so long as it is high enough to ensure fluid-like behavior of the first resin composition and low enough not to have a substantial evaporation of the vaporizable plasticizer component in the first resin composition. Therefore the temperature may be selected to be any value by varying the type and the amount of the vaporizable plasticizer component in the first resin composition. While any suitable elevated temperature may be employed, preferred temperatures are in the range at least about 30xc2x0 C. to about 200xc2x0 C.
The step of comminuting the first resin composition is typically carried out by further subjecting the dispersion of the first resin composition in the organic medium at an elevated temperature. The comminuting temperature may be selected to be any value so long as it is high enough to ensure fluid-like behavior of the first resin composition and low enough not to have a substantial evaporation of the vaporizable plasticizer component. Therefore the temperature may be selected to be any value by varying the type and the amount of the vaporizable plasticizer component in the first resin composition. While any suitable elevated temperature may be employed, preferred temperatures are in the range at least about 30xc2x0 C. to about 200xc2x0 C. However, it needs not be the same temperature as the dispersion temperature. The shearing required for the comminuting step of the present invention is substantially smaller due to the presence of the vaporizable plasticizer component compared to that for the process without a vaporizable plasticizer component. Effective comminution may be obtained in a vessel containing a 10 cm radius impeller-type agitator and with the agitator rotation speed as low as 100 rpm.
The step of removing the vaporizable plasticizer component from the comminuted resin composition is typically carried out by maintaining the mixture of the resin component and the organic medium at an elevated temperature close to or above the boiling temperature of the vaporizable plasticizer component. Under such conditions, the vaporizable plasticizer component evaporates from the comminuted particulate resin composition and subsequently from the processing vessel. The process may be more expeditiously carried out when the vaporizable plasticizer composition is immiscible with the organic medium. The removal step is stopped when the vaporous effluent from the process vessel does not show a trace of the vaporizable plasticizer component.
The steps of dispersion, comminution and removal of the vaporizable plasticizer component may be conducted in distinctive and discontinuous steps, sequentially in a single vessel or in a series of overlapping steps in a single vessel.
The step of recovering the comminuted resin particles is carried out by first cooling the content of the process vessel below the glass transition temperature of the resin component and subsequently by filtering solid resin particles from the organic medium. Any suitable filtration equipment may be used. Subsequently, dry resin particles are obtained by washing the filtered particles with a low boiling organic solvent such as isohexane and drying off the wash solvent at a temperature below the glass transition temperature of the resin component.
In another aspect of the present invention, there is provided a particulate resin composition comprising resin particles that are substantially spherical in shape, have an average diameter of from about 1 to about 10 microns, and have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8, prepared by comminuting a precursor composition comprising a vaporizable plasticizer component in an organic medium under shear at an elevated temperature wherein the particles are substantially insoluble in the organic medium. The resin component may be a polyester resin or a styrene copolymer resin.
In yet another aspect of the present invention, there is provided a particulate resin composition comprising resin particles that are substantially spherical in shape, have an average diameter of from about 1 to about 10 microns, have a uniform and narrow size distribution with a span value less than 1.0, more preferably, with s span value less than 0.8 and further have an irregular surface texture characterized by the surface roughness index greater than 1.2. The surface roughness index is defined as the ratio of surface areas of the irregular textured particles and smooth texture particles and is discussed in more detail hereinafter.
In still yet another aspect of the present invention, there is provided a particulate resin composition comprising a polyester resin component and an optional charge control agent wherein the particles are substantially spherical in shape, have a volume average diameter in the range of from about 1 to about 10 microns, have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8. The particles have an irregular surface texture characterized by the surface roughness index greater than 1.2 wherein the polyester resin component includes a polyester resin having a weight average molecular weight of about 100,000 g/mol or less.
In a still further aspect of the present invention, there is provided a articulate resin composition comprising a styrene copolymer resin component and an optional charge control agent wherein the particles are substantially spherical in shape, have a volume average diameter in the range of from about 1 to about 10 microns, have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8. The particles have an irregular surface texture characterized by the surface roughness index greater than 1.2 wherein the styrene copolymer resin component includes a styrene copolymer having a weight average molecular weight of about 100,000 g/mol or less. Particularly preferred styrene copolymer resins include copolymers of styrene and acrylate as well as copolymers of styrene and butadiene.